Silicoaluminophosphates (SAPOs) have been used as adsorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. The use of SAPOs in conversion of oxygenates to light olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities. Catalysts deactivate during use due to various reasons. Some catalysts are sensitive to exposure to moisture while other catalysts are impacted by the operating conditions in a reactor. In the methanol to olefins (MTO) process, for example, SAPO-34 catalyst deactivation can be divided into two types: 1) short term deactivation due to coking; 2) long term deactivation due to hydrothermal aging. The short term deactivation can be completely reversed by careful coke burning. Activity lost during hydrothermal aging is much more difficult to recover. The present invention provides a method for rejuvenating SAPO catalysts.
As is known in the development of new large scale, commercial production facilities in the commodity chemical business, many problems arise in the scale up from laboratory and pilot plant operations. Although some work has been published relating to the intermediate activities in the catalyst production-to-use chain, few of the problems associated therewith have been addressed. For example, U.S. Pat. No. 4,681,864 to Edwards et al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability, and that stability can be improved by using a particular activation process. In this process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400° to 800° C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425° C. The heating process is first carried out in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the sieve.
Briend et al., J. Phys. Chem. 1995, 99, 8270-8276, teaches that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data are presented which suggest that over at least the short term, this crystallinity loss is reversible. Even over a period of perhaps two years, the data suggests that crystallinity loss is reversible when certain templates are used.
Several patents assigned to ExxonMobil Chemical Company have addressed a discovery that activated SAPO molecular sieve will exhibit a loss of catalytic activity when exposed to a moisture-containing environment—both from the ambient humidity as well as the high temperature steaming environment exposed to the catalyst in the conversion of oxygenates to olefins. These patents have addressed several different methods for recovery of catalyst activity. In U.S. Pat. No. 6,639,117, the freeze drying of the catalyst is employed to recover the catalyst activity. In U.S. Pat. Nos. 6,498,120 and 6,825,391, a silicoaluminophosphate molecular sieve is rejuvenated through contact of the catalyst with an anhydrous liquid or vapor, in particular methanol. Another method is disclosed in U.S. Pat. No. 6,756,516 in which the catalyst is treated with certain organic nitrogen compounds to protect the catalyst from degradation through exposure to moisture.
Although it may be possible to use the above techniques to protect a catalyst from the exposure to moisture within an ambient atmosphere, there still remains the need to rejuvenate catalyst that has been deactivated from exposure to high temperature steam environments. Surprisingly, in light of the observations by ExxonMobil and the other prior art regarding the deactivation of SAPO catalysts by exposure to moisture, it has now been discovered that water or ammonium salts can be used to rejuvenate deactivated catalyst, especially SAPO-34 that has been deactivated by high temperature steam.